194 CRUSTACEA. AMPHIPODA. 



antennas, one of mandibles, two of maxillae, and one of 

 maxillipedes. The thorax has seven, and the abdomen 

 six, pairs of appendages. The eyes are sessile. There 

 are two sub-orders, (1) Amphipoda, (2) Isopoda. 



SUB-ORDER. AMPHIPODA. 



The Amphipods (e.g. Gammarus, Talitrus) are usually 

 of small size and the body is laterally compressed. There 

 are seven segments in the thorax, the appendages of which 

 bear the gills. The abdomen is elongated and bears six 

 pairs of appendages, the three anterior serve for swimming, 

 the three posterior for jumping. Some of the Amphipods 

 are marine, others fresh water ; the fossil forms are rare, a 

 few doubtful examples occur in the Palaeozoic ; those found 

 in the Tertiary are mainly fresh water genera. 



SUB-ORDER. ISOPODA. 



The body is flattened dorso-ventrally. In the thorax 

 there are six or seven segments, each with a pair of 

 ambulatory appendages. The abdomen is often short, 

 and is formed of six segments (frequently fused) and a 

 telson ; some of the appendages function as gills. Many 

 Isopods are marine, others fresh water, whilst a few live 

 on land ; the fossil forms are rare. A specimen, named 

 Prearcturus, from the Old Red Sandstone of Herefordshire, 

 has been referred to this group, but two undoubted 

 examples occur in the Solenhofen Slates ; in England the 

 earliest form (with the possible exception of Prearcturus) 

 is Archceoniscus from the Purbeck Beds. 



